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Note that at the time of CreateInstanceData, like other shading plugins, Bxdfs are unable to inspect the values of pattern network inputs; therefore, in cases these inputs are provided (i.e: RixParameterList::GetParamInfo() returns k_RixSCNetworkValue) the Bxdf may have no choice but to defer inspection of the inputs until BeginOpacity() or BeginInterior(). At that time, those methods may then choose to return NULL instead.

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• RixBxdf::GenerateSample() to generate samples of the bxdf function, one sample for each point of the shading context.;
• RixBxdf::EvaluateSample() to evaluate the bxdf function, one direction for each point of the shading context;
• RixBxdf::EvaluateSamplesAtIndex() to evaluate the bxdf function, one-or-many directions for a given point of the shading context;
• RixBxdf::EmitLocal() to retrieve the bxdf's local emission.

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The RixBxdf methods above are expected to return quantities for each point of the shading context originally given to RixBxdfFactorRixBxdfFactory::BeginScatter(), excepting RixBxdf::EvaluateSamplesAtIndex(), that operates on a single point of the shading context, evaluating the bxdf function for one or many directions.

The GenerateSample() function has the following input parameters: transportTrait, lobesWanted, and random number generator. The transportTrait tells the Bxdf the subset of light transport to consider: direct illumination, indirect illumination, or both. lobesWanted specifies what lobes are  are requested, for example specular reflection, diffuse transmission, etc. The random number generator should be called to generate well-stratified samples; such samples typically reduce noise and improve convergence compared to using uniform random samples.

The GenerateSample() function has the following output parameters (results): lobeSampled, direction, weight, forward pdf, reverse pdf, and compTrans.  lobeSampled is similar to the input lobesWanted, and specifies which lobe was  was actually sampled. direction (Ln) is the generated ray direction vectors; these directions must have unit length. weight is a color per sample indicating that sample's weight. The forward pdf should account for light moving from the L to V direction where as the reverse pdf account for the opposite (from V to L). Bxdfs should always provide both pdf values for the integrators to use. compTrans is an optional result which can be used to indicate transmission color; this will be used as alpha in compositing. A bxdf should check that compTrans is not NULL before assigning to it.  All results are arrays with one value per sample point.

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In order to maintain physical correctness, bxdfs are expected to conserve energy and obey the Helmholtz reciprocity. Care should be taken so that RixBxdf::GenerateSample(),   RixBxdf::EvaluateSample() and RixBxdf::EvaluateSamplesAtIndex() return consistent results. This allows bxdf plugins to be compatible with different rendering techniques such as:

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